ORGANIC DEVELOPEMENT. 611 



formance of their proper offices, receiving into 

 their cavities, through a tube temporarily pro- 

 vided for that purpose, the fluid of the yelk, and 

 preparing nourishment from it. 



In the mean time, early provision is made for 

 the aeration of the fluids by an extensive but 

 temporary system of vessels, spread over the 

 membrane of the egg, and receiving the influ- 

 ence of atmospheric oxygen through the sub- 

 stance of the shell, which is sufficiently porous 

 to transmit it ; and these vessels, being brought 

 into communication with the circulatory system 

 of the chick, convey to its blood this vivifying 

 agent. As the hings cannot come into use till 

 after the bird is emancipated from its prison, 

 and as it was sufficient that they be in readiness 

 at. that epoch, these organs are among the last 

 which are constructed : and as the mechanism 

 of respiration in this class of animals does not 

 require the play of the diaphragm, this muscular 

 partition, though begun, is not completed, and 

 there is no separation between the cavities of the 

 thorax and the abdomen. 



The succession of organic metamorphoses is 

 equally remarkable in the formation of the 

 diversified apparatus for aeration, which is re- 

 quired to be greatly modified, at different periods, 

 in order to adapt it to different elements : of this 

 we have already seen examples in those insects 

 which, after being aquatic in their larva state, 



